The present invention relates to a high frequency electrical connector for a cable of the type having a signal conductor and a ground shield, such as a coaxial cable.
Separable coaxial to coaxial cable terminations are expensive due to construction techniques and materials required by high bandwidth (typically three to six GHz) applications. Typically available separable coaxial cable terminations are also unsuitable as "high density" interconnects because their relatively large circular cross sections, dictated by impedance control and signal propagation considerations, prevent dense signal line spacing. As a result of these disadvantages of separable coaxial to coaxial terminations, and as a result of the need for high density interconnects in current high performance applications, a connector which provides a cost and performance compromise has been developed between coaxial cable terminations and high density "signal-ground-signal" interconnects. This connector is referred to as a shielded controlled impedance (SCI) interconnect.
An SCI interconnect is typified by the 1.times.2 single coaxial cable connector described in U.S. Pat. No. 5,184,965. In the '965 patent, a sheet metal shield box encloses one signal socket contact and one ground socket contact, each designed to mate with a header pin. The ground socket makes electrical contact with the inside of the metalized shield box via a spring arm to provide continuity from the header pin through the shield box to the coaxial cable shield. By grounding the shield box internally and in only one location, the cross-sectional bulk of a typical symmetrical coaxial cable termination is eliminated.
As the need for greater density of the interconnects has increased, SCI connectors having an increased number of signal contacts have been introduced. FIGS. 1(a)-1(d)illustrate such an SCI connector. In a prior art design shown in FIGS. 1(a)-(d), two coaxial cables share a common ground contact within the connector (referred to as a 1.times.3 connector) to provide a higher density interconnect than two 1.times.2 connectors stacked together (e.g., one less ground pin is required). Like the 1.times.2 SCI connector described above, each signal line is fully enclosed by shielding.
The 1.times.3 SCI connector as illustrated in FIGS. 1(a)-1(d) is constructed from many components in a labor intensive manner. For example, the shield boxes 2 for each signal connector (not shown) are formed separately and held together by a welded spring plate 4 in front and a solder bridge 6 in back. In addition, the ground pin contact 8 is attached to only one of the separate shield boxes, thereby possibly increasing self-inductance and cross talk within the connector. Additional components, such as an adjacent box grounding contact 9 must be attached as separate components by welding or soldering. As can be seen, the connector requires a labor intensive and expensive assembly process. In addition, the large number of individual components leads to a greater likelihood of connector failure or poor performance due to improper assembly.
A high frequency cable connector for coaxial or twinaxial shielded cables is shown in U.S. Pat. No. 5,632,634. The '634 patent provides a high density electrical connector for coaxial or twinaxial cables, where the connector has an outer shield that may be electrically connected to a ground pin in a mating connector. The connector provides at least two inner insulating housings separately surrounded by an outer shielding member. The inner insulating housings have inner signal contacts, and the outer shielding members are commonly grounded by way of a grounding spring clip positioned between the outer shielding members.
The connector described in the '634 patent, while providing electrical shielding to the connection, is not capable of providing the same characteristic impedance for all signal lines. In particular, the distance between the ground return path and each of the signal conductors is not equal. That is, the outer signal conductors are further from the ground return path than are the inner signal conductors. This means that the signal conductors do not experience a uniform impedance across the connector, and any signals traveling through the connector will experience degradation as a result. Further, the grounding spring clip of the '634 patent is not positioned for controlling the impedance of the connector. Specifically, the grounding spring clip does not make contact with the outer shield member near the front edge of the connector. Rather, the grounding spring clip contacts the shield member well behind the front edge of the connector. This makes the ground return path of the connector much longer than the signal path through the connector, thereby causing an increased self-inductance and increased impedance within the connector.
The above prior art connectors do not provide adequate performance characteristics for high performance systems. Inadequate performance characteristics include, for example, excessive "ground bounce", the inability to control the impedance in the connector without significant discontinuities, or to provide connector bandwidth equal to the system in which the connector is used. For example, any difference in the lengths of the signal path and the ground path through the connector causes increased self-inductance in the connector, and hence an increase in ground bounce. Ground bounce refers to the transient voltage appearing across a portion of a signal return path when return currents from rising or falling signals pass through areas of significant inductance. This transient voltage results in signal degradation and crosstalk. It is therefore advantageous to position the grounding contacts of the connector as close as possible to the engagement point of the grounded component, e.g., the ground pin of the mating pin header. In this manner, the lengths of the signal and ground paths are kept as close as possible to the same length, thereby minimizing any self-inductance within the connector, and also minimizing the impedance variation within the connector.
It is also important to minimize any variation in the distance separating the signal path and its ground return path as the signal moves through the connector. If the spacing between the signal path and the ground path varies from one signal path to another, the signal line will experience a different impedance, thus causing degradation in the system using the connector. Such impedance variations limit the bandwidth of the connector and are not acceptable in many high performance systems. Other factors important in providing a low impedance current return include the width of the current return path. That is, a current return path having a larger cross section is preferred over one with a smaller cross section.
It is apparent that what is needed is a high density, high frequency connector for shielded cables that provides an improved performance and ease of assembly over currently available connectors, and which provides a low self-inductance ground return path.